Angiogenesis, the sprouting of new capillary blood vessels from existing vasculature, is a complex biological process of critical importance to the treatment of numerous pathologies and the success of tissue engineering. Amongst the many promising strategies to promote angiogenesis in tissue engineering includes the controlled delivery of pro-angiogenic molecules with precise spatial and temporal control. An alternative approach is to deliver an appropriate cell type that can provide a more physiologic mixture ofpro- angiogenic cues that will accelerate the recruitment of host vessels. In conjunction with two senior co- investigators, the PI has begun to explore this latter strategy in a robust, fibrin-based 3-D in vitro model of capillary morphogenesis. Using this model system, which supports the formation of stable (>3 weeks) capillary-like structures with well-defined lumens, we have shown that capillary morphogenesis is significantly inhibited by increasing fibrin matrix density. Furthermore, we have shown that this fibrin denisty block is partially due to the inability of endothelial cells to upregulate a subset of matrix metalloproteinases (MMPs) required to locally remodel the matrix and sustain capillary invasion. However, the addition of bone marrow-derived mesenchymal stem cells (MSCs) to the 3-D tissue construct significantly enhances capillary morphogenesis, partially overcoming the inhibition caused by increased matrix density by upregulating the expression and/or activity of this subset of MMPs. Building on these preliminary findings, this new investigator Bioengineering Research Grant proposal seeks to test the hypothesis that MSCs can stimulate angiogenesis within dense 3-D fibrin matrices both in vitro and in vivo. These objectives will be achieved via three specific aims as follows: 1.) Quantify the ability of MSCs embedded throughout dense fibrin matrices to enhance capillary-like network formation in vitro. 2.) Probe the roles played by a subset of MMPs (MMPs-2 and -9 and the membrane-type MT1-MMP) in the angiogenic enhancement by MSCs. 3.) Determine the ability of MSCs to stimulate vascularization in vivo in a subcutaneous site in a SCID-mouse model. Successful completion of these studies will contribute to our fundamental understanding of the role of the ECM and MMPs in angiogenesis, and will ultimately impact the design of synthetic ECM analogs and the use of stem cells in tissue engineering applications.